1. Field of Invention
This invention relates to an examination method and apparatus utilizing nuclear magnetic resonance (hereinafter called "NMR") for externally determining a distribution of certain atomic nuclei or the like within a body being examined; and more particularly, to an improved NMR imaging apparatus for use in medical equipment.
2. Description of Prior Art
The principles of nuclear magnetic resonance are well known, and may be explained with reference to FIGS. 1(a), 1(b), 2(a), 2(b), 3(a) and 3(b). An atomic nucleus comprises protons and neutrons (one proton in case of hydrogen) which are regarded as rotating with angular momentum of nuclear spin I. FIGS. 1(a) and 1(b) show an atomic nucleus of hydrogen (.sup.1 H) As shown in FIG. 1(a), the atomic nucleus comprises one proton P which rotates with a spin quantum number 1/2. Since the proton P has a positive charge e.sup.+ as shown FIG. 1(b), there is a magnetic moment M. Therefore, each atomic nucleus of hydrogen can be regarded as one small magnet.
FIGS. 2(a) and 2(b) schematically illustrate such a magnetic property of atomic nuclei. In the case of a ferromagnet1c material such as iron, small magnets represented by atomic nuclei are oriented uniformily as shown in FIG. 2(a), so that the atomic nuclei, as a whole, exhibit magnetization. With respect to hydrogen, however, small magnets (magnetic moments) are directed at random, as illustrated in FIG. 2 (b), and fail to exhibit magnetization.
When the material, such as hydrogen or the like, is placed in a static magnetic field H.sub.0 applied in the direction of Z, all atomic nuclei are oriented in the direction of H.sub.0. In other words, the nuclear energy levels are quantized in the direction of Z.
FIG. 3(a) shows the manner in which atomic nuclei of hydrogen are oriented in a static field. As the spin quantum number of hydrogen is 1/2, the energy levels are divided into two energy levels of -1/2 and +1/2, as shown in FIG. 3(b), with the energy difference .DELTA.E therebetween being expressed by the following equation: EQU .DELTA.E=.gamma.h H.sub.0 ( 1)
wherein .gamma. is the gyromagnetic ratio; h=h/2.pi.; h=Planck's constant. Each atomic nucleus is subjected to the force M.times.H.sub.0 due to the static field H.sub.0, and hence revolves about the Z axis in processional movement with an angular velocity given by the following equation: EQU .omega.=.gamma.H.sub.0 (Lamour angular velocity) (2)
When the system under such motion is subjected to an electromagnetic wave (normally a radio frequency wave) having a frequency corresponding to the angular velocity .omega., resonance occurs, and the atomic nucleus absorbs an amount of energy corresponding to the energy difference .DELTA.E expressed by the equation (1) and is transferred to the higher energy level. Different kinds of atomic nuclei with angular momentus of nuclear spin have different gyromagnetic ratios .gamma., and, therefore, resonate with respective different frequencies. As a result, resonance of desired atomic nuclei of a certain element can be ascertained. The quantity of atomic nuclei which exist can be determined by measuring the intensity of resonance. Those atomic nuclei which have been transferred to the higher energy level will return to the lower energy level upon elapse of a period of time determined by a time constant called "relaxation time".
Relaxation times are classified into a spin-lattice relaxation time (longitudinal relaxation time) T.sub.1 and a spin-spin relaxation time (transverse relaxation time) T.sub.2. Data on a material distribution can be obtained by observing the relaxation times. In solids, generally, spins are substantially fixed in given positions over the crystal lattice, so that the spins tend to act mutually. Thus, the relaxation time T.sub.2 is short and the energy produced by nuclear magnetic resonance is first given well through the spin system, and then to the lattice system.
Accordingly, the time T.sub.1 is much longer than the time T.sub.2. In liquids, molecules move freely, and energy exchange between spins and that between spins and the molecule system (lattice) take place with substantially the same ease. Thus, the times T.sub.1, T.sub.2 are approximately equal to each other. The time T.sub.1 in particular is a time constant dependent on the manner in which the compound molecules are coupled. It is known that the time T.sub.1 with a malignant tumor, for example, is substantially different from the Time T.sub.1 with, for example, a normal tissue.
Although NMR has been described above with reference to hydrogen atomic nuclei (.sup.1 H), the same measurements can be achieved with other atomic nuclei having angular momentums of nuclear spin, such as the atomic nuclei of phosphorus (.sup.31 P), carbon (.sup.13 C), sodium (.sup.23 Na), fluorine (.sup.19 F), oxygen (.sup.17 O), and other elements.
Since the quantity of certain existing atomic nuclei and their relaxation times can be measured, various kinds of examinations of an object body can be made by obtaining various items of chemical information about particular atomic nuclei contained with a material.
There has been proposed, conventionally, an NMR examination apparatus which operates on the same principle as that of an X-ray CT (computerized tomograph) by exciting protons in a hypothetical section of a body being examined, obtaining an NMR resonance signal corresponding to each projection for many directions across the body, and determining the intensity of the NMR resonance signal in each position of the body through a reconstructive method.
FIG. 4 depicts waveforms of signals of an examination process used in a conventional apparatus. First, an object to be examined is subjected to a x-gradient magnetic field Gz.sup.+, as indicated in line (b) of FIG. 4, and RF pulses (90.degree. pulses) having a narrow fequency spectrum, such as shown in line (a) of FIG. 4. At this time, protons only in the plane in which the Lamour angular velocity is given by below equation (3) are excited. EQU .omega.=.gamma.(H.sub.0 +.DELTA.Gz) (3)
Magnetization M has its direction shifted through 90.degree. into alignment with the y' axis if expressed on a coordinate system, such as shown in FIG. 5(a), as it rotates with the angular velocity .omega.. Then, an x-gradient magnetic field Gx and a y-gradient magnetic field Gy are applied as shown in FIG. 4, line (c) and line (d), to produce a two-dimensional gradient magnetic field for detecting an NMR resonance signal (FID signal=Free Induction Decay signal). Since the magnetization M is scattered gradually in the directions of the arrows within the x'-y' plane, as depicted in FIG. 5(b), the NMR resonance signal is reduced until it is eliminated upon elapse of a time Ts as shown in FIG. 4, line (e). By subjecting the NMR resonance signal thus obtained to a Fourier transform, a projection is obtained which is perpendicular to a gradient magnetic field which comprises the x-gradient magnetic field Gx and the y-gradient magnetic field Gy.
Upon elapse of a given period of time Td, a next sequence is repeated in the same operation as described above. In successive sequences, Gx and Gy are gradually changed. NMR resonance signals can thus be obtained in many directions across the object body for respective projections.
With the conventional apparatus thus described, the time Ts in which the NMR resonance signal entirely disappears, ranges from 10 to 20 mS, and the time Td required for transition to a next sequence is about 1 sec. because of the relaxation time T.sub.1. Therefore, provided that one body sectional plane is to be reconstructed with 128 projections, for example, the measurement requires at least two minutes. This is a substantial obstacle to high speed operation.
Also known in the art is a technique known as the DEFT process, which means Driven Equilibrium Fourier Transform, reported in Journal of the American Chemical Society, 91:27, Dec. 31, 1969, pages 7784-7785. This DEFT process has been proposed for use in NMR analyzers. However, no where in the art has there been any disclosure wherein the DEFT process has been utilized in or even suggested for use in the NMR imaging apparatus.
Briefly, the DEFT process uses a pulse sequence for high speed operation which comprises, for example, (90.degree. x . . . .tau. . . . 180.degree. y . . . .tau. . . . 90.degree. -x . . . Td).sup.n.
The following discussion is not a part of the prior art, but represents theoretical investigations leading up to and including the invention. For example, when effecting two dimensional imaging with the DEFT process, 90.degree. pulses would tend to excite protons within a particular slice plane with a selective excitation method (a gradient field being simultaneously applied), and no problem arises out of this procedure. The 180.degree. pulse would excite protons with both selective and non-selective methods.
FIG. 23 illustrates the results, as simulated by a computer and using the Bloch equations, of a distribution of magnetization Mz on the z axis immediately prior to the first 90.degree. pulse in the direction of a sliced thickness. The 90.degree. pulse is subjected to Gaussian modulation for selective excitation. The results were computed by using average T.sub.1, T.sub.2 and Tr=100 mS (repetitive time) of a living body. Mz is assumed to be 1 prior to execution of the pulse sequence and has a magnitude corresponding to an NMR signal intensity.
With non-selective 180.degree.-pulse for the DEFT process, Mz outside of the slice plane is quite small, as indicated by the dot and dash line A in FIG. 23.
A multi-slice method has generally been employed in which during the wait time Td for a pulse sequence, identical pulse sequences are successively applied to other plural slice planes, and after Mz has been subjected to longitudinal relaxation time T.sub.1 due to the sufficiently long Td during that time, a view next to the first slice plane is obtained. This method is effective as a quasi-high speed method since the NMR signal (magnitude of Mz) is prevented from being reduced, and at the same time, data on a plurality of planes can be attained. However, the multi-slice method requires that Mz outside of the slice plane be large without being affected by excitation in other slice planes.
The above requirement leads to the shortcoming wherein the DEFT process using non-selective 180.degree.-pulses cannot be used with the multi-slice method since Mz outside of the slice plane is small. An actual slice configuration is expressed by Mz of FIG. 23, as multiplied by a slice shape function (Gaussian type in the illustrated example), and is shown in FIG. 24.
There is no problem with selective 180.degree.-pulses for the DEFT process since Mz outside of the slice plane is large as indicated by the dash line B in FIG. 23. However, the slice shape is disadvantageous in that it has three peaks as shown in FIG. 24. This is because, upon application of selective excitation 180.degree.-pulses, the magnetization M in a slice interface acts in a complex manner to make vector directions of M different from each other, with the result that the signal is reduced.
As above described, the conventional DEFT process, as it now exists, is not suitable for use in the NMR imaging apparatus.